60 DINOCERATA. 



In this comparison, moreover, of the extinct forms with those of more 

 modern time, inclnding recent mammals, it may be taken for granted tliat 

 the brain-cavity of the extinct forms, as well as of those now living-, was 

 entirely tilled b}' the brain ; since, with a possible single exception, no 

 mammal is known in which this is not the case. 



In comparing the size of the brain in manunals with tliat of reptiles 

 and fishes, an important point to be borne in mind would be the fact, that 

 in the two latter classes the brain-cavity is not always entirely filled by 

 the brain. Tlio present comparison deals with mammals alone, and this 

 restriction is here of no importance. 



The fact that among existing mammals there, are some anomalous 

 features in the size of the brain in allied grou])s has not been forgotten, 

 but such instances, even if they occurred among extinct mammals, would 

 not materially aifect the general comparison here proposed. 



In the following pages a series of figures is given, showing the 

 comparative size of the brain, and its position in the skull in a number 

 of the larger ungulate mammals, recent and extinct. To make the 

 comparison a fair one, the skulls are all dr;i\\n of the same absolute 

 size, thus showing, at once, the relative proportion of the brain in each. 

 The skulls are placed lioi'izontally, the plane of the molar teeth being as a 

 rule taken as a base. In the case of the foiu- artiodactyls, figures 80 to 

 83, page 67, this position has been somewhat changed, as, in this group 

 of ungulates, the brain is more or less inclined backward in the skull. In 

 these cases, the skull is represented as somewhat inclined forward, thus 

 raising the posterior part of the brain. The angle of inclination of the 

 face and of the brain, is made equal, thus giving to both the best position 

 for comparison, and not materiall}- aftectiug it for the present jjurpose. 



A striking illustration of the development of the brain from the early 

 Tertiary to the present time may be seen in figures 63-65, page 61, where 

 is shown, first, the skull of Dinoceras, the largest land mammal of the 

 Eocene, with the brain in position ; second, the skull of the gigantic 

 Miocene Brontotheriuni, with the brain also in position; and third, the skull of 

 the recent horse. Otlier comparisons, equally striking, can readily be made. 



